The use of ozone in many applications involving sterilising and cleaning air is well known. Ozone generating devices have been designed for a great variety of domestic and industrial applications. All depend on ozone's great oxidising potential to kill micro-organisms and oxidise other organic particles and materials. Depending on the application, ozone is generated by means of ultraviolet radiation or electrical discharge to convert atmospheric oxygen to triatomic ozone, which can be highly effective at destroying organic atmospheric contaminants. Ozone is, however, highly toxic at high concentrations and it is increasingly clear that even at much lower concentrations it is irritant, being particularly linked with asthmatic complaints in those chronically exposed to it. In many territories there are strict statutory limits on the concentration of ozone to which members of the public and employees at a place of work may be exposed. In the UK, the Health and Safety Executive recommendation (EH38) is that the exposure limit to ozone should be 0.1 ppm (0.2 mg m−3) as an 8-hour time-weighted average concentration, with a short-term exposure limit of 0.3 ppm (0.6 mg m−3) as a 15-minute time-weighted average concentration.
Although undoubtedly effective at high concentrations, there is considerable evidence that ozone is ineffective as a biocide or in oxidising organic contaminants at concentrations that are safe for chronic human exposure (Dyas et al, 1983, J Clin Pathol 36: 1102-1104; Berrington and Pedlar, 1998, J Hosp Infect 40: 61-65; Esswein et al, 1994, Appl Occup Environ Hygiene 9: 139-146). Such effect as it has in reducing odours is, in many cases, probably a mere masking with its own characteristic smell.
Alternative approaches to removing micro-organisms and other small airborne organic particles, such as smoke, obviously include direct filtration of the air. Various type of filter including so-called High Efficiency Particulate Air (HEPA) filters (defined as removing 99.97% of particles of 0.3 micron size) and electrostatic HAF (High Airflow, electret) filters capable of similar performance at higher airflows are commonly used. Although effective in some situations, such filters suffer from the disadvantages that trapped (and potentially infective) material remains on the filters, necessitating frequent changes of filter and remaining a hazard until the filters are replaced. This is a particular problem where the air being filtered is humid. In addition, such filters are incapable of removing small viral particles.
In addition to the abovementioned problems, conventional filtration systems used in aircraft air supply systems in which cabin air is repeatedly recycled with a small proportion thereof being replaced by fresh air from outside the aircraft, present particular problems, due to the extreme temperature ranges encountered. Usually such filtration systems employ HEPA filters mounted in mesh cages. One significant limitation of HEPA filters is the relatively low face speed thereof, i.e. the maximum airflow speed across the face of the filter at which particles of a given size can be trapped by the filter. The filter material frequently becomes damp which significantly increases the resistance to the airflow, thereby substantially increasing the load on the air circulation drive, which in turn substantially increases the fuel consumption of the aircraft. This problem is even further aggravated by freezing of the damp filter material at the very low temperatures encountered at typical airliner cruising altitudes. A further consequence of such freezing is that it can result in buckling and distortion of the mesh cage which in turn often results in jamming of the filter cartridge inside the cartridge mounting thereby increasing maintenance downtime and cost.
In my earlier GB 2358350 there is disclosed a proposed hand drier apparatus using a corona unit with quartz glass and stainless steel mesh electrodes and operating at 9 mA and 4 kv in order to eject a stream of ozone at the user. Further research has disclosed, however, that the proposal as disclosed in this publication would be quite impractical—quite apart from exposing the user to dangerous levels of ozone.
Thus there remains a need for an efficient means of removing airborne pollutants such as organic particles, micro-organisms and odours from air in forced airflow circulation systems, without release of potentially hazardous levels of ozone into an enclosed environment, and whilst minimizing air flow resistance.